Recovery of sulfur dioxide contained in waste gases



April 20, 1954 H. F. JoHNsToNE RECOVERY 0F SULFUR DIOXIDE CONTAINED IN WASTE GASES Filed Jan. 26, 1953 5 Sheets-Sheet l HTTORNEYS 3 Sheets-Sheet 2 MOFQMOLIZm Illa H. F. JOHNSTONE RECOVERAY OF SULFUR DIOXIDE CONTAINED IN WASTE GASES lApril 20, 1954 Filed Jan- 26, 1955 April 20, 1954 H. F. JoHNsToNE RECOVERY OF SULFUR DIOXIDE CONTAINED IN WASTE GASES Filed Jan- 26, 1953 3 Sheets-Sheet 3 I IN VEN TUR. Henry F. Jo hnstqne WWW HTTORNEYS Patented Apr. 20, Vv193,4

RECOVERY OF SULFUR DIOXIDE CONTAINED IN WASTE GASES Henry F. Johnstone, Urbana, Ill., assigner to Texas Gulf Sulphur Company, New York, N. Y., a corporation of Texas Application January 26, 1953, Serial No. 333,039

Claims.

This invention relates to the recovery oi' sulfur dioxide from gases and relates especially to the recovery of sulfur dioxide from waste gases such as those resulting from the combustion of sulfur containing coal or from the roasting or sintering of sulfur-containing materials as in ore rening processes.

The recovery of sulfur dioxide from waste gases is commercially important not only from the standpoint of minimizing pollution problems but also from the standpoint of conservation of sulfur resources. At the present time there is a prevailing shortage of sulfur resulting from increased demand and diminishing resources of brimstone in the United States. On the other hand, it has been estimated that the amount of sulfur as sulfur dioxide which is emitted to the atmosphere in waste combustion gases exceeds the total quantity of sulfur consumed. It is apparent, therefore, that the sulfur dioxide contained in such waste gases offers a potential source of sulfur whose utilization is a matter of great commercial significance provided `that recovery of the sulfur in a useful form may be accomplished by methods and equipment such that the cost for thereby producing a sulfur-containing product of sulfur-containing products is competitive with or less than the cost of producing such product or products from other sources of sulfur. The nature of the sulfur-containing product or products produced is also a signiiicant factor not only from the standpoint of the inherent commercial value of the product or products in question but also from the standpoint of availability of markets therefor without entailing unduly high shipping costs.

The sulfur dioxide contained in waste gases from power plants exceeds that in all other forms of waste gases, but in the waste gases from power plants the sulfur dioxide content is relatively low, the proportion being dependent principally upon the sulfur content of the coal consumed. An increasing percentage oi the coal consumed in power plants contains more than 4% sulfur and using current combustion methods the stack gases from the combustion of such coal vcontains about 0.4% by volume of sulfur dioxide or a somewhat higher percentage. One of the objects of this invention is, therefore, to provide an improved process whereby sulfur dioxide can be successfully recovered in a practical and economically feasible manner from such waste gases or even from waste gases containing a substantially lesser content of sulfur dioxide such as 0.1% or 0.2% by volume. However, the process of this invention likewise is of utility in the recovery of sulfur dioxide from waste gases which contain higher proportions of sulfur dioxide such as those containing about 0.7% to about 0.8% by Volume of sulfur dioxide which are produced in 2 a lead sinterng operation or those containing about 5% to about 6% by volume of sulfurdioxide produced in a zinc roasting operation. More generally, any Waste combustion gases containing a substantial proportion of sulfur dioxide provide a potential source for the recover;7 of sulfur dioxide therefrom according to this invention.

O'f the possible absorbent-,s which may be utilized for the absorption 4of sulfur dioxide from waste gases, ammoniacal solutions have certain distinct advantages due to the fact that, because of the high order of solubility of the ammonium compounds produced by absorption of sulfur dioxide?, the ammoniacal solution may be used in such concentrations as to have a large capacity for absorbing sulfur dioxide from the waste gases. When sulfur dioxide is absorbed by an ammoniacal solution sulfur dioxide may react with the ammonia contained in the solution to form ammonium sulte or ammonium bisulfite and ordinarily the result of the absorption is to produce both ammonium sulte and ammonium bisulnte, the relative proportions thereof being deherein and in the claims the term ammonium' pendent on such factors as the concentration of the ammonia in the ammoniacal solution which is available for reaction with sulfur dioxide, the concentration of the sulfur dioxide in the waste gases that are contacted with the ammoniacal solution, the temperature at which the absorption is carried out and the extent to which equilibrium conditions are approached in carrying out the absorption. For convenience in reference sulte compound is used generally as applicable to ammonium suli'lte, to ammonium bisuliite and to ammonium sulte-bisuliite mixtures.

The relative proportion of ammonium sulte and of ammonium bisulte in a solution of ammonium sulfite compound produced by the reaction oi sulfur dioxide in waste gases with ammonia in an ammoniacal solution contacted therewith may be expressed in terms of the ratio of concentration of the available ammonia, i. e., ammonia available for reaction with the sulfur dioxide, to the concentration of the sulfur dioxide, each of those concentrations being in terms of mols per mols of Water. The concentration of the available amonia in terms of mols of ammonia per 100 mols of water is designated herein as Cs and is to be distinguished from the concentration of the total ammonia which, as expressed in terms of mols per 100 mols of water, is designated herein as Ct. Under the conditions prevailing during oxidation of sulfur dioxide by an ammoniacal solution some yof the sulfur dioxide becomes oxidized to sulfur trioxide which in the aqueous medium becomes converted to sulfuric acid that in turn vreacts with ammonia in the ammoniacal solution to produce ammonium sulfate, and to the extent that part of the ammonia in the ammoniacal solution becomes converted to ammonium sulfate, such quantity of the total ammonia (Ct) is rendered unavailable for forming a solution of ammonium sulte compound from which sulfur dioxide may be regenerated in a concentrated usable form. To the extent that ammonium sulfate is so produced there is a recovery of sulfur dioxide from the original stack` gases in the form of ammonium sulfate, which is a product having commercial value. However, as will be explained more fully hereinbelow, one of the objects of this invention is the recovery of sulfur dioxide from Waste gases and the regeneration of a high proportion of the so-absorbed sulfur dioxide for the production of sulfur dioxide as an end product in a concentrated and usable form.

The concentration of the sulfur dioxide in terms of mols of sulfur dioxide per 100 mols of Water that is absorbed by the ammoniacal solution by its reaction with ammonia to form ammonium sulte compound, as distinguished from ammonium sulfate, is designated herein as S. If ammonium sulte compound is in the form of ammonium sulte, (NH4) zSOs, it is apparent that there are two mols of ammonia per mol of absorbed sulfur dioxide and in such case the value of the ratio S/Ca is 5. On the other hand for ammonium bisulfite, NHil-ISOS, the value of the ratio S/Ca is l. Ordinarily, under the conditions for the absorption or sulfur dioxide from waste gases by contacting the Waste gases with an ammoniacal solution, the proportion of ammonium sulfite to ammonium bisulfite preferably is such that the value of the ratio of S/Ca is in the neighborhood of about .7 to about .9 or even somewhat higher, depending on the concentration of the sulfur dioxide in the Waste gases, on theconcentration of the ammonia in the solution, and on the temperature of the solution.

' If sulfuric acid is added to a solution of ammonium sulfite compound that results from the absorption of sulfur dioxide contained in waste gases by an ammoniacal solution, the sulfuric acid reacts with the ammonium sulte compound to produce ammonium sulfate and sulfur dioxide and the sulfur dioxide can be readily liberated from the resulting solution of amonium sulfate. After the sulfur dioxide has been separated from the ammonium sulfate solution, the ammonium sulfate may be produced in crystalline form by evaporating Water from the solution, as for example, in an evaporator-crystallizer wherein the evaporation of the water is accelerated by heating the solution. If the solution is of suicient concentration oi ammonium sulfate, this compound may be recovered by simply cooling Without evaporation of Water. The ammonium sulfate thus produced includes that which results from reacting the ammonium sulfite compound with the added sulfuric acid together with such ammonium sulfate as is produced because of oxidation of some of the sulfur dioxide in the waste gases during absorption by the ammoniacal solution. It is apparent that in such an operation all of the ammonia introduced into the system is contained in the recovered ammonium sulfate.

While the recovery of the sulfur dioxide from a solution comprising ammonium sulfite compound may be accomplished by acidification of the solution with sulfuric acid there are certain disadvantages incident to an operation of this type. Due to Vthe large consumption of ammonia and its ultimate conversion to ammonium sulfate it is apparent that an operation of this type is commercially feasible only if the ammonia used is obtainable at a relatively low cost and if the ammonium sulfate can be disposed of on a competitive basis as regards current market price and shipping cost. Moreover, the amount of sulfur dioxide which can be recovered as such is relatively low in relation to the amount of ammonia consumed and the amount of ammonium sulfate which necessarily is produced. Thus, in a typical operation of this type about 3 moles of ammonia are consumed in the operation in order that about 2 mols of sulfur dioxide may be liberated by acidication with sulfuric acid. Moreover, approximately 2 mols of sulfuric acid are required in order to react with the ammonium sulte compound to produce about 3 mols of sulfur dioxide. Since the sulfuric acid used in the system in order to liberate the sulfur dioxide ordinarily is produced in a sulfuric acid plant from the sulfur dioxide formed by reaction of the sulfuric acid with ammonium sulte compound it is apparent that for each 3 mols of sulfur dioxide resulting from the acidification of ammonium sulte compound with sulfuric acid about 2 mols of the sulfur dioxide so released are reintroduced into the system by conversion to the sulfuric acid employed in the acidiction step. It is also apparent that the cost of the sulfuric acid plant and the conversion of sulfur dioxide to sulfuric acid therein is a substantial item in the overall economy of the system. When one takes into consideration the proportion of the sulfur dioxide liberated by acidification which is converted to sulfuric acid for use in the acidiiication step the residual proportion of sulfur dioxide which is recoverable as such from the system will only be about l mol for each 2 mols of ammonium sulfate produced. This is a disadvantage if a relatively high yield of sulfur dioxide is desired.

Sulfur dioxide can also be recovered from a solution of ammonium sulte compound obtained by absorbing sulfur dioxide contained in Waste gases by means of an ammoniacal solution by a stripping operation utilizing heat which has the effect of raising the vapor pressure of the sulfur dioxide in the solution so that it may be separated from the solution along with water vapor which is also separated from the solution as the result of the increase in the vapor pressure of the Water at the elevated temperature appropriate for stripping. Such stripping is ordinarily referred to as steam stripping due to the presence of the Water vapor that is released along with the sulfur dioxide. In such steam stripping the so- -lution may be heated by any suitable means such as steam. Ordinarily if steam is used to heat the solution it is directed into out-of-contact heat exchange relation with the solution so that the solution'may be heated wtihout unduly diluting the solution and without unduly increasing the amount of water vapor which is released from the solution along with the sulfur dioxide. However, heating by direct contact with steam could be used.

At ordinary absorption temperatures the vapor pressure of the ammonia in the ammoniacal solution is relatively low. However, when the solution is heated to stripping temperature the vapor pressure of the ammonia is also increased. In order to effect the stripping of the sulfur dioxide from the solution it is necessary that, during the stripping operation, the vapor pressure of sulfur dioxide be greater than the vapor pressure of ammonia. The vapor pressure of the sulfur dioxide in relation to that of the ammonia is dependent upon the concentration o the sulfur dioxide in the solution in relation to the ammonia, and the concentration of the sulfur dioxide in the solution is fixed by the concentration of the sulfur dioxide in the waste gases and the temperature at which the sulfur dioxide is absorbed from the waste gases by the ammoniacal solution. In the case of relatively dilute gases this places a limitation on the amount of sulfur dioxide Which may be absorbed in the ammoniacal solution in relation to the ammonia. In any case, the maximum amount of sulfur dioxide that is recoverable by stripping is the difference between the concentration oi the sulfur dioxide in the solution that may be obtained in equilibrium with the Waste gases at the absorption temperature and the concentration of the sulfur dioxide in the solution at which its vapor pressure is just greater than that of the ammonia in the solution when the solution is at the stripping temperature. It is seen, therefore, that there is a severe limitation on the amount of sulfur dioxide that can be separated from a solution of ammonium sulnte compound by resort to the heating of the solution from the absorption temperature to the stripping temperature. Moreover, for simple steam stripping the minimum ratio of steam to sulfur dioxide that is separated from the solution at the stripping temperature cannot be less than the molar ratio of Water vapor to sulfur dioxide at the wet bulb temperature of the original gases and ii' the solution comes approximate ly to equilibrium with the Water vapor and sulfur dioxide in the original gases the ratio of the vapor pressures of the water and of the sulfur dioxide remains approximately constant as the temperature is raised. Thus, the heat requirement for eiecting stripping not only is that required to heat the solution from absorption temperature to stripping temperature, but also that which is required for separating a necessarily relatively large proportion of Water vapor from the solution along with the sulfur dioxide. The result is that the requirements for heat in effecting a simple stripping operation are so high as to oiset to the extent of impracticality the value of the sulfur dioxide which may be recovered in this Way. Moreover, in the recovery oi sulfur dioxide by steam stripping there is a tendency for the amount of ammonium sulfate formed in the solution as the result of oxidation of sulfur dioxide during the scrubbing of the Waste gases to build up with consequent lowering of the capacity of the ammoniacal solution for effectively absorbing sulfur dioxide and some special means for counteracting this tendency has to be resorted to.

According to this invention the diihculties and disadvantages above mentioned which are incident to the recovery of sulfur dioxide from a solution of ammonium sulte compound produced by absorption of sulfur dioxide contained in waste gases by means of an ammoniacal solution have been overcome by combining in a single operation both acidification With sulfuric acid and steam stripping. By the combined processing steps above mentioned it is possible to greatly reduce the supply of both ammonia and sulfuric acid into the system as a Whole as compared with acidification alone and it is also possible to greatly reduce the amount of heat required as compared With steam stripping alone. Moreover, it is possible to obtain a higher yield of sulfur dioxide in relation to ammonium sulfate as compared with the acidification process and substantial nexibility is afforded as between the relative amounts of sulfur dioxide and ammonium sulfate which are produced so that the practice of this invention may be varied according to the relative values of the sulfur dioxide recovered and the ammonium sulfate recovered depending on such factors as the current market values of these products and the location of the plant in relation to available markets therefor. Moreover, it is possible to control and hold at a low value the amount of ammonium sulfate contained in the scrubbing solution during absorption of the sul fur dioxide from the waste gases.

According to this invention a portion of the ammonium sulte compound which is comprised in the effluent solution obtained by absorbing sul fur dioxide contained in waste gases by means of an ammoniacal solution is caused to react with sulfuric acid to form ammonium sulfate and sulfur dioxide by the addition of sulfuric acid to eiuen-t solution containing the portion of the ammonium sulte compound which is reacted with the acid. The sulfur dioxide so produced is separated from the residual solution containing the ammonium sulfate, and it is collected and recovered. The ammonium sulfate contained in the residual solutionmay be recovered as byl causing it to crystallize therefrom and then collecting and recovering the ammonium sulfate crystals. roln a second portion oi the ammonium suliite compound contained in the eluent solution sulfur dioxide is stripped by heating effluent solution containing the second portion of the ammonium sulfite compound thereby separating sulfur dioxide from residual ammoniacal solution. The sulfur dioxide which is thus produced by stripping is separated and collected and the residual ammoniacal solution from which the sulfur dioxide has been stripped is recycled so as to be comprised in the ammoniacal solution directed into contact with the Waste gases.

Preferably in the absorption of the sulfur dioxide contained in the Waste gases by ammoniacal solution the ammoniacal solution is directed into contact with the waste gases in at least tWo Zones through which the gases are sequentially passed from a rst zone to a second zone and the concentration of the ammonia in the ammoniacal solution that is available for reaction With the sulfur dioxide in the gases is maintained so as to be Substantially different in the different zones with decrease in the concentration of the available ammonia in the direction of the sequential travel of the Waste gases through the Zones so that the concentration of the ammonia contained in the iirst Zone is substantially greater than the concentration of the ammonia contained in the ammoniaoal solution in the second zone. In such case in the practice of this invention residual ammoniacal solution Which results vfrom having stripped sulfur dioxide and water vapor therefrom during the stripping step above described is recycled so as to be comprised predominantly in ammoniacal solution that is directed into contact with the waste gases in the rst Zone. During the stripping operation both Water vapor and sulfur dioxide are evolved and the sulfur dioxide is separated from 'the water vapor by condensing the water vapor. Also comprised in the water vapor and sulfur dioxide which are separated during the stripping operation is a small lamount of ammonia which reacts with a corresponding small proportion of the sul'- fur dioxide during the condensation of thewater vapor to form a small amount of ammonium bisulte in the condensed water vapor. The bull: of the sulfur dioxide is separable from the condensate and can be recovered for use in acid manufacture or for any other suitable purpose. The resulting condensate containing a small amount of ammonium fbisulfite may then be recycled so as to be comprised predominantly in the ammoniacal solution which is directed into contact with the waste gases in the above mentioned second Zone wherein a lesser concentration of available ammonia is maintained as compared with the first zone.

i The practice of this invention may be varied as regards the point in the system at which a portion or the ammonium sulfite compound contained in the effluent solution from the scrubber is acdiiied with sulfuric acid in relation to the stripping of sulfur dioxide from another portion of the ammonium suliite compound comprised in the eii'luent solution by steam stripping. Thus, the eluent solution may be divided into two portions, one of which is regenerated by steam stripping and the other of which is acidiiied with sulfuric acid for the recovery of sulfur dioxide and ammonium sulfate. In such case it is Ordinarily preferable to subject the major portion of the effluent from the scrubbing operation to steam stripping.

Alternatively, the effluent solution may be divided into two portions t0 one of which a portion of the total amount of the sulfuric acid that is used in the overall operation is added prior to steam stripping. To the other portion of the eiuent solution there is separately added the balance of the sulfuric acid and the system is so controlled that the amount of ammonium sulfate produced in any period of time by oxidation of the sulfite, by absorption of sulfuric acid vapor from waste gases, and by the addition of sulfuric acid to the first portion of the eiluent solution prior to stripping is removed in the same period of time in the second portion to which the balance of the sulfuric acid is added for liberating sulfur dioxide and producing additional ammonium sulfate. In this modification of the process the addition of sulfuric acid to the portion of the effluent solution which is stripped has the effect of greatly increasing the vapor pressure of the sulfur dioxide so as to facilitate the separation of the sulfur dioxide by stripping and thus reduce the amount of heat required for effecting the stripping operation.

According to a third modiiication of this invention the entire eluent from the scrubbing operation is acidied with an amount of sulfuric acid which is suiiicient to react with only a portion of the ammonium sulte compound contained in the solution and the acidified solution is then passed to the stripper wherein the sulfur dioxide is stripped from the ammonium suliite compound which remains after the partial acidication of the total ammonium suliite compound contained in the solution by sulfuric acid. Thus, while the eiiluent solution is sequentially treated as a Whole iirst by acidification and then by subjecting it to stripping, the operation is similar to the other two modifications of this invention hereinabove referred to in that it involves the reacting of a iirst portion of the ammonium sulfite compound contained in the eiuent solution with sulfuric acid and the stripping of sulfur dioxide from a second portion of the ammonium sullte compound comprised in the effluent solution. After the solution has been both acidied and subjected to steam stripping the residual ammoniacal solution containing ammonium sulfate can be treated for the separation of ammonium sulfate therefrom merely by cooling the solution so as to cause crystallization of the ammonium sulfate. Upon removing the crystallized ammonium sulfate from the solution the residual ammoniacal solution is recycled so as to be comprised in the ammoniacal solution which is directed into contact with the waste gases. According to this modification the effect of adding all the sulfuric acid prior to stripping is such as to greatly increase the vapor pressure of the sulfur dioxide and thereby facilitate its release so that the heat requirements are still less than in the case of the other modifications above mentioned.

So as to aliord a better understanding of this invention certain examples of the practice thereof are described hereinbelow for illustrativepurposes in connection with the accompanying drawings wherein,

Fig. 1 is a iiow sheet illustrating the practice of this invention wherein the eiiiuent from the scrubber is separated into two portions, one of which is subjected to steam stripping and the other of which is subjected to acidification.

Fig. 2 is a flow sheet illustrating the practice of this invention wherein the effluent from the scrubber is divided into two portions, one of which is subjected to both acidification and steam stripping and the other of which is subjected to acidiiication; and V Fig. 3 is a iiow sheet illustrating the practice of this invention wherein the entire eluent from the scrubber is subjected sequentially to acidication and to steam stripping.

For purposes of illustration and comparison the operations described hereinbelow in connection with Figs. l, 2 and 3 are With reference to the recovery of sulfur dioxide from waste combustion gases which have been subjected to cooling and rehumidincation so that the Wet bulb temperature is about 90 F. The gases contain 0.44% of sulfur dioxide by volume and contain 0.03 pounds of water per pound of dry gas.

Referring to Fig. l the waste gases are introduced by the gas conduit Il! into the scrubber which is indicated generally by the reference character Il and from the top of which they are passed by the gas line I2 to a stack for discharge to the atmosphere. Preferably, as aforesaid, the waste gases are broughtJ into contact with ammoniacal solution in a plurality of zones. For purposes of illustration the scrubber Il has been shown as comprising the three zones I la, l ib and llc. The waste gases pass sequentially through the zones lla, IIb and llc. Each zone of the scrubber is such as to provide intimate contact between the waste gases and ammoniacal solution which is caused to flow through the respective Zones. Any suitable construction may be employed for providing intimate contact between the gases and the ammoniacal solution in the respective Zones of the scrubber. For example, each of the zones may be of the gridpacked tower construction whereby the ammoniacal solution is caused to come in intimate contact with the waste gases and in counterflow thereto in each zone of the scrubber. In each zone of the scrubber the liquid rate of the ammoniacal solution in relation to the ow of Waste gases therethrough may, for example, be three gallons per 1,000 cubic feet of gas. Howeverl for better indicating the molar relationships of the interacting chemical substances the following description of illustrative examples of the practice of this invention is in terms of mols and in terms of concentrations expressed as mols per 100 mols of water, the mol quantities stated below indicating -Elow quantities in terms of mols per unit of time.

The waste gases which enter the scrubber by the gas line i9 are such as to introduce into the system 190 mols of inert gas, 9.5 mols of water in the form of water vapor and 0.88 mol of sulfur dioxide. The system is designed so as to absorb 80% of the sulfur dioxide from the gases with the result that 0.176 mol of sulfur dioxide is passed from the scrubber II by the gas line I2. The gases leaving through the line I2 contain the same amount of inert gas and water vapor as the entering gases. namely, 190 mols of inert gas and 9.5 mols of wat-or vapor. According to the present example the system is such that 50% of the sulfur dioxide contained in the entering gases is absorbed in zone Ila of the scrubber while and 5% of the sulfur dioxide contained in the entering gases are absorbed in zones Hb and. Hc respectively. This differential absorption of sulfur dioxide in the different zones is alforded by causing the concentration of the available ammonia in the ammoniacal solution to be different in the different zones of the scrubber, such concentration of the ammonia being greatest in Zone Ha and least in zone Ilc, while an intermediate concentration of the available ammonia is maintained in zone I lb, as will be more particularly described hereinbelow. The term available ammonia as used herein refers to the ammonium ions in the ammoniacal solution which are available to form ammonium sulte or ammonium bisulte by reaction with sulfur dioxide in the waste gases. The available ammonia includes the ammonium ions which are combined with sulfur dioxide in the form of ammonium sulte or ammonium bisulte but does not include ammonium ions which have become inactive as the result of the formation of ammonium sulte or possibly some other salt or salts other than ammonium sulilte or bisulfite. As mentioned above the symbol Ca indicates the concentration of the available ammonia in terms of mols per 100 mols of water. The symbol Ct refers to the total ammonia including both the available ammonia and any ammonia that has become unavailable as the result of having become combined in the form of ammonium sulfate, for example.

In each of the zones or the scrubber il the ammoniacal solution is caused to be circulated therethrough so that in the entering solution and in the leaving solution there will be 100 mols of water relative to the mols of the constituents of the waste gases in contact therewith in the different zones. Accordingly the concentrations below mentioned, which are in terms of mols per 100 mols of water, indicate the content in mols of the constituents of these solutions as well as the flow quantities in terms of mols per unit of time. Considering scrubber I l as a whole, the concentration of available ammonia (Ca) in the solution entering Zone llc by line i3 is 2.2 mols per 100 mols of water, the total ammonia (Ct) being 2.62 mols per 100 mols Water. The sulfur dioxide content of the ammoniacal solution in the form of ammonium sulilte compound is indicated herein as S, and as the ammoniacal solution enters the zone ile in Contact with the waste gases in this zone the sulfur dioxide content (S) is 2.06 mols per 100 mols water, thus establishing the Value of the ratio S/Ca as .937. The sulfate content (which is indicated by (A)) is .21 mol per 100 mols of water. In the elluent line I4 the concentration of available ammonia (Ca) is 18.27 mols per 100 mols water, the total ammonia (Ct) is 25.35 mols per 100 mols water, the sulfur dioxide (S) is 116.26 mols per 100 mols water, the Value of the ratio S/Cs is .89, and the sulfate (A) is 3.54 mols per 100 mols water.

in each of the zones of the scrubber II the bulk of the ammoniacal solution is recirculated. Thus, with reference to zone llc, 90.9% of the eluent taken therefrom by the line I5 is directed by the line It for reentry in zone llc through the line i3 and 9.1% 0f the effluent is directed into the zone I Ib of the scrubber by the lines I1 and It. To make up for the 9.1% of solution directed from zone l Ic te zone I Ib, 1 mol of water is fed into the system by the line I9 and mixed with 8 mols of water returned to the scrubber by the line 20 from the regenerator portion of the system to be described hereinbelow, the resulting mixture being directed by the line 2i for entry into zone Hc by line I3. This solution returned to the scrubber by the line 29 contains .165 mol of sulfur dioxide and .166 mol of ammonia. .45 mol of ammonia is fed into the system from the ammonia supply source 22 by the line 23, and since the ammonia is comprised in a 36% by weight aqueous ammonia solution, .8 mol oi water is also fed into the system from this source. 15 of the aqueous ammonia that is fed into the system from the source 22 is directed by the line 24 so as to be commingled with the solution entering the zone I Ic.

90.5% of the solution leaving zone IIb by the effluent line 25 is returned to this Zone by lines 26 and IB, and 9.5% of this eilluent is directed to the zone IIal by the lines 27 and 23. 55% of the aqueous ammonia Yfrom the aqueous ammonia supply source 22 is also caused to enter Zone IIb by the lines 23, 29, 30 and I8.

60% of the solution leaving the zone I la by the effluent line it is returned thereto by the lines 9! and 29, and 29% of the aqueous ammonia from the aqueous ammonia supply source 22 is also directed into the Zone l la by the lines 23, 29, 32 and 29. The balance of the solution entering the zone I Ia is supplied thereto by the line 33 which feeds ammoniacal solution into the line 3l from the regenerator portion of the system to be described below. The ammoniacal solution fed into the zone Ila by the lines 33, 3! and 2.9 supplies 30.2 mols of water and the constituents and the concentrations thereof contained in this solution will be given hereinbelow.

While the waste gases enter the scrubber il at about F., the conditions of the reaction are such that the temperatures prevailing in the zones lla, Hb and llc are F., 93 F. and 91 F., respectively. The vapor pressure in millimeters of mercury of the sulfur dioxide in the gases as they enter the scrubber II by the gas line I0 is 3.34 mm. The vapor pressure of the sulfur dioxide contained in the gases is 1.67 mm. as they leave zone Ila, 0.835 mm. as they leave zone IIb, and is .668 mm. as they leave zone I lc. The value of the partial pressure of sulfur dioxide for equilibrium with the ammoniacal solution in each of the zones i Ic, i Ib andY Ile is substantially below that of the sulfur dioxide in the Waste gases in each of the zones so that absorption of sulfur dioxide takes place readily in each l l of the zones. The value of the partial pressure of ammonia for equilibrium with the ammoniacal solution which enters zone IIc, in contact with the leaving gases is only about .001 mm., so that ammonia losses to the stack gases are slight. namely, about 0.001 mol.

40% of the solution leaving the scrubber I I by the line I4, is directed from the scrubber portion of the system by the line 3'4 for recovery of sulfur dioxide therefrom. The solution contained in the line 34 is split into two portions. One portion constituting 38.2% of the solution leaving the scrubber by effluent line I4 is directed by the line 35 for regeneration by stripping sulfur dioxide therefrom. The balance of the solution constituting 1.8% of the solution leaving the scrubber by the line I 4 is directed by the line 36 for acidication vvith sulfuric acid and resultant liberation of sulfur dioxide and formation of ammonium sulfate.

The portion of the solution contained in line 35 is rst partially heated in the heat exchanger 31 in out-of-contact heat exchange relation with residual solution which is directed from the stripper 38 by the aforesaid line 33 which feeds this residual solution back into line 3| and thence into Zone I Ia of the scrubber by line 28. The partially heated solution is directed from the heat exchanger 3T! to the heat exchanger 39 by the line 40 to be further heated as with steam supplied by lines 4I and 42 in out-of-contact heat exchange relation with the solution in the heat exchanger 39. The heated solution is then directed by the line 43 to the stripper 38 wherein the desired stripping temperature of about 230 F. is maintained as by the steam coil 66 to which steam is fed by the lines 4I and 6l. The stripper 38 is maintained at atmospheric pressure.

8 mols of Water in the form of vapor are directed from the stripper 38 by the line 44 and carried therewith is the stripped sulfur dioxide, namely, 0.518 mol, along With a small amount of ammonia, namely, 0.166 mol. This vaporous mixture is taken by the line 44 to the ammonia absorber 45 with which the condenser 46 is connected by the vapor and condensate lines 4'! and 48, respectively. In the ammonia absorber 45 the .166 mol of ammonia immediately reacts with a corresponding molar quantity of sulfur dioxide to form ammonium bisulfite, and the 8 mols of Water condensate carrying the .166 mol of sulfur dioxide and .166 m01 of ammonia are recycled, as aforesaid, by the line 26 so as to be comprised in the ammoniacal solution entering zone I Ic of the scrubber preferably after having passed through the cooler '68 in the line 20 for the purpose of cooling the condensate.

The sulfur dioxide separated from the water vapor in the condenser 46, namely, 0.352 mol, is taken from the condenser 46 by the line 48 and is collected and recovered in concentrated form by any suitable means as indicated by the reference character 49.

As the result of the separation of water vapor, along with the stripped sulfur dioxide and a small amount of ammonia from the solution entering the stripper 38, residual ammoniacal solution is produced which, as aforesaid, is returned by the line 33 so as to be comprised in the ammoniacal solution directed into contact with the Waste gases in zone IIa of the scrubber. The line 38 may contain the heat exhanger 3l and the cooler 50, although the cooler 50 may be omitted. In this residual solution the ammonia will have become concentrated so that the concentration therein of the available ammonia (CF.) is 22.9 mols per 100 mols of water. The concentration of sulfur dioxide in this solution (S) is 19.1 mols per 100 mols of water, the concentration of sulfate (A) is 4.54 mols per 100 mols of water, and the value of the ratio S/Ca is` .834.

In the scrubber II about 9% of sulfur dioxide recovered from the waste gases, namely, about .063 mol, becomes oxidized to sulfur dioxide which in the presence of the aqueous ammonia becomes converted to ammonium sulfate. Under the conditions above described the sulfate content is, as aforesaid, 3.54 mols per 100 mols of Water in the solution leaving the scrubber i I by the line I4, and such being the case by taking ofi 1.8% of this eiiluent solution through the line 3% the ammonium sulfate so formed is removed from the system at the rate that it is produced. rlhe sulfur dioxide comprised in the portion of the eiliuent solution carried in the line 36, whose concentration is 16.26 mols per 100 mols of Water, is released therefrom by mixing with the solution 0.162 mol of sulfuric acid which is added through line 5I from the sulfuric acid supply 52A and which reacts with the ammonium sulflte compound comprised in the solution to form sulfur dioxide and ammonium sulfate. Separation of the sulfur dioxide from the solution is promoted by heating the solution in the stripper 53 as by means of the steam coil 54 to which steamsteam is fed by the lines 55 and 53. In the evapo rator-crystallizer 60 1.3 mols of Water are re moved, this being the amount of water fed into the system by the line I9 (1 mol) and from the ammonia supply source 22 by the line 23 (0.8 mol). The crystallized ammonium sulfate, 0.225 mol, may be removed from the evaporator-crystallizer by any suitable means as indicated by the line 64 to a recovery vessel or the like that is indicated at 65.

In the foregoing operation, of the sulfur dioxide recovered by absorption from the waste gases of the sulfur dioxide content of the waste gases), 9% becomes converted to sulfate, 50% is released by the stripping step, and 41% is released by the acidification step. The sulfuric acid required, considering the system as a whole, is 25 mols per 100 mols of sulfur dioxide produced which is substantially less than half the amount of the sulfuric acid which would be required if 91% of the recovered sulfur'dioxide were to have been released by sulfuric acic?L acidification alone. The amount of ammonium sulfate produced is 0.66 ton per ton of sulfur dioxide recovered from the waste gases, which amount is less than halfA the amount of ammonium sulfate that would be produced if the 91% of the recovered sulfur dioxu ide were to have been released solely by acidification with sulfuric acid. As a corollary is also' seen that the requirementr of the foregoing system for supply of fresh 'ammonia thereto is less than half of the requirement for supply of fresh ammonia if the recovered sulfur dioxide were to have been released solely by acidification with sulfuric acid. The loss of ammonia to the stack gases is only 0.2 pound per ton of sulfur dioxide recovered.

The proportion of the recovered sulfur dioxide that is obtained as sulfur dioxide instead of as ammonium sulfate likewise is much higher as compared with release of the sulfur dioxide solely by acidification with sulfuric acid. Moreover, because of the much reduced requirements for sulfuric acid supply to the system the expense for providing and maintaining a sulfuric acid plant whereby the supply of sulfuric acid may be maintained is correspondingly reduced.

The steam required for supplying heat to the system is relatively low as compared with regeneration of the scrubber eiiluent solely by steam stripping. Thus, in terms of pounds of steam required per pound of sulfur dioxide recovered the steam requirements are 0.33 pound for the preheater 39, 3.8 pounds for the stripper 38 and 0.75 pound for the evaporator-crystallizer 60, the total being 4.88 pounds. This results from the fact that 41% of the sulfur dioxide recovered from the waste gases is liberated by acidication and that it is necessary to liberate only 50% of the sulfur dioxide by steam stripping and in so doing to heat to the stripping temperature only 38.2% of the effluent leaving the scrubber Il by the eluent line I4. Moreover, as compared with regeneration of the eiiluent from the scrubber solely by steam stripping, the system above described solves the problem of preventing build up of ammonium sulfate as the result of oxidation of sulfur dioxide in the scrubber and the recovery of the ammonium sulfate so produced in crystallized form is accomplished in an economical way. Under ordinary conditions it is desirable to remove about 2% by volume of the efuent leaving the scrubber by the line lil to accomplish this purpose and to treat this portion of the eiiiuent with sulfuric acid as has been described above, the objective being that the proportion of the scrubber eilluent which is treated with sulfuric acid shall be such that its rate of withdrawal from the system shall be at least sufficient to carry with it the amount of sulfate that is formed or absorbed in the other portions of the system per unit of time. However, if a higher yield of ammonium sulfate is desired, or if the amount of sulfate that is formed or absorbed in other portions of the system is larger than that above exemplified, a larger proportion of the elluent can be so removed and acidied with sulfuric acid to form ammonium sulfate and sulfur dioxide by reaction with the ammonium sulte compound comprised therein. Alternatively if the amount of sulfate that is formed or absorbed in other portions of the system is less than that above exemplified the pro portion of scrubber eiuent that is acidiiied with sulfuric acid may be substantially less.

According to the practice of this invention as illustrated and described hereinbelow in connection with Fig. 2, the operation is in general like that illustrated and described hereinabove in connection with Fig. 1 and utilizes essentially the same elements of equipment. Accordingly the elements of equipment shown in Fig. 2 which correspond to those shown in Fig. 1 have been indicated by the same reference characters and their description will not be repeated. The principal difference between the system of Fig. l and that of Fig. 2 is that in Fig. 2 the portion of the effluent solution from the scrubber Il which is subjected to stripping is partially acidied with sulfuric acid before it enters the stripper 38. The sulfuric acid used for this purpose is supplied from the source 69 by the line 10 so that it becomes commingled with the solution in the line 43 before the solution enters the stripper 38. While the amount of sulfuric acid introduced from the source 69 is suicient to react with only a portion of the ammonium sulte compound in the solution that enters the stripper 38, its addition acts to greatly increase the partial pressure of the sulfur dioxide comprised in the solution whereby its separation by stripping may be more readily accomplished with the result that the system illustrated in Fig. 2 involves substantially less requirements for heat supplied as compared with the system above described in connection with Fig. 1 while maintaining essentially the same requirements respecting the amounts of ammonia and total sulfuric acid supplied to the system and the amounts of sulfur dioxide and ammonium sulfate produced. Thus, for purposes of comparison the system shown in Fig. 2 is exemplified below in connection with the recovery of sulfur dioxide from waste gases which as they enter the scrubber Il are of the same composition and at the same temperature as stated hereinabove in connection with the system of Fig. 1, namely, 190 mols inert gas, 9.5 mols water, 0.88 mol sulfur dioxide, at a temperature of 90 F. Also, as in connection with the system exemplified in connection with Fig. 1, the exit gases leaving the scrubber Il by the gas line I 2 contain 190 mols inert gas, 9.4 mols water, 0.176 mol of sulfur dioxide, and 0.001 mol of ammonia. Of the of the sulfur dioxide recovered from the entering gases 50% is absorbed in zone lla of the scrubber, 25% is absorbed in zone IIb and 5% is absorbed in zone i Ic. The vapor pressures of the sulfur dioxide in the waste gases at the several points with reference to the scrubber are also the same as hereinabove mentioned in connection with Fig. 1. The temperatures maintained in the zones lla, l Ib and llc are likewise substantially the same.

According to the example described hereinabove in connection with Fig. 2, 0.45 mol of ammonia is supplied to the system in the form of a 36% aqueous ammoniacal solution and the total amount of sulfuric acid introduced into the system is 0.162 mol. Since the amount of sulfur dioxide which is oxidized so as to produce ammonium sulfate in the scrubber is also the same, namely, 9% of the sulfur dioxide recovered, it also follows that the amount of ammonium sulfate recovered from the system also is the same as that in connection with the example described with reference to Fig. 1, namely, 0.225 mol. In connection with the system exemplified in connection with Fig. 2 the percentage of sulfur dioxide which is released solely by reaction of a separated portion of the eiiluent with sulfuric acid is less, namely, 30%, while a larger percentage of the recovered sulfur dioxide, namely, 61%, is released during the stripping step.

While there are the foregoing similarities between the systems as exemplified in connection with Figs. 1 and 2 respectively, nevertheless, there are differences as regards the details of the operation which are occasioned primarily from the fact that the addition of a portion of the sulfuric acid to the portion of the solution which is subjected to stripping results in such increase in the partial pressure of the sulfur dioxide that the required stripping is accomplished with incidental separation from the solution subjected to stripping of a decreased amount of water as water vapor. By thus raising the partial pressure of the sulfur dioxide, higher concentration of ammonium sulfate may be employed and decreased concentration of available ammonia, whereby a higher equilibrium vapor pressure of the solution leaving the scrubber is permitted and resultant decreased heat requirements for electing stripping are afforded. The introduction of some of the sulfuric acid into the system that is subjected to stripping also requires the acidication of a somewhat larger separated portion of the effluent from the scrubber so as to prevent excessive build up of ammonium sulfate in proportion "of the eiiluent solution which is regenerated by stripping sulfur dioxide therefrom.

With further reference to the process exemplified in connection with Fig. 2 the concentration of available ammonia (Ca) in the solution entering zone I Ic of the scrubber by the line I3 is 0.85 mol per 100 mols of water, the total ammonia (Ct) being 1.59 mols per 100 mols of water. The sulfur dioxide content (S) of the ammoniacal solution which enters the zone IIc in contact with the Waste gases in this zone is 0.76 mol per 100 mols of water. It follows from the foregoing that the ratio of S/Ca is 0.892. 94.6% of the solution leaving zone I lc by the effluent line I5 is recirculated into this zone by the lines I6 and I 3 and 5.4% of the solution leaving the zone IIc is directed into the zone IIb by the lines II and I8. In order to make up for the 5.4% of the solution leaving the zone Ilc which is directed into the zone IIb, 4 mols of water from line 20 are combined with 1.3 mols of Water from the line I9, the resultant 5.3 mols of water being fed into the zone llc by the lines 2| and I3. 15% of the aqueous ammonia fed into the system from the ammonia supply source 22 is also fed into the y zone IIc by the lines 23, 24 and I3. Y

94.2% of the solution leaving the zone lib by the line 25 is recirculated into the zone I lb by the lines 26 and I8. 55% of the aqueous ammonia fed into the system from the ammonia supply source 22 is also fed into the zone I lb by the lines 23, 29, 30 and 18.

5.8% of the solution leaving the zone I Ib is directed into the zone I Ia by the lines 21 and 28 and are mixed with 30% of the ammonia supplied to the system from the ammonia supply source 22 by the lines 23, 29 and 32 and with residual ammoniacal solution which is returned to zone IIa of the scrubber from the stripper 38 by the lines 33-and 3| and which supplies 97.9 mols of Water containing dissolved ammonium sulte compound and ammonium sulfate in concentrations which will be mentioned hereinbelow.

Under the foregoing conditions the solution leaving the zone Ela of the scrubber comprises 100 mols of water, as is also the case in connection with the effluents from zones I Ib and I Ic. In the effluent from zone IIa the concentration of available ammonia (Ca) is 10.6 mols per 100 mols of water, the concentration of the total ammonia (Ct) is 21.6 mols per 100 mols of water, the concentration of the sulfur dioxide is mols per 100 mols of water, the concentration of sulfate is 5.52 mols per 100 mols of Water and the value v ofthe ratio S/ Ca is .950.

scribed in connection with the example of this invention described with reference to Fig. l. However, before this portion of the solution enters the stripper 38 0.052 mol of sulfuric acid is drected from the sulfuric acid supply source 69 by the line i0 so as to be adrnixed therewith in the line d3. The temperature and pressure in the stripper are 225 F. and atmospheric respectively, and 0.451 mol of sulfur dioxide is stripped therefrom along with 0.019 mol of ammonia. Due to the acidification of the solution in the stripper 38 the above mentioned amount of sulfur dioxide is carried from the stripper with only 4 mols of water which, as aforesaid, is returned to zone I IC of the scrubber by the line 20 after having been condensed in the condenser 59 which is asso` ciated with ammonia absorber 45. The 4 mols of water which are returned by the line 20 carry with it the 0.019 mol of ammonia together with 0.019 mol of sulfur dioxide which is combined therewith as ammonium bisulte. 0.432 mol of sulfur dioxide is separated from the condensed Water and is directed by the line 'II to the collecting means indicated at 49.

The residual ammoniacal solution from the stripper 38 is returned to zone I Ia of the scrubber by the line 33 as above mentioned. In this solution the concentration of the available ammonia (Cs) is 10.89 mols per mols of water, the concentration of sulfur dioxide (S) is substantially 10 mols per 100 mols of water, the concentration of the sulfate (A) is 5.81 mols per 100 mols of water and the value of the ratio S/Ca is 0.917.

The remaining portion of the eiiluent solutionfrom the scrubber, namely, 2.1% thereof is treated for the liberation of sulfur dioxide there-4 from by reaction with sulfuric acid. 0.11 mol of sulfuric acid is directed from the sulfuric acid' source 52 by the line 5I so as to become commingled With the solution in the line 3B prior to introduction of the solution into the stripper 53. 0.209 mol of sulfur dioxide is separated from the solution in the stripper 53 and is directed by the line 5'! into suitable means for recovering the sulfur dioxide which is indicated at 58. The above mentioned quantity of solution which is directed to the stripper 53 by the line 35 is sufficient to carry with it such ammonium sulfate as is formed in the rest of the system as the result of oxidation of sulfur dioxide in the scrubber and as the result of the sulfuric acid which is added to the portion of the eliluent solution that is directed to the stripper 38. The quantity of ammonium sulfate thus produced together with the additional ammonium sulfate that is produced by the addition of sulfuric acid from the sulfuric acid source 58 to the solution" entering the stripper 53 is crystallized in the evaporatorcrystallizer 60 lwith evaporation of 2.1 mols of Water therefrom, namely, the same as the amount of water that is supplied to the system by line l5)v (1.3 mols) and from the ammonia supply source 22 (0.8 mol). The yield of 0.225 mol of ammonium sulfate may be collected in the collecting means indicated at 65. In this modification of the practice of this invention it is desirable under normal conditions to remove about 2% by volume of the eliluent leaving the scrubber by the line I Il in order to prevent an excessive build up of arnmonium sulfate in other portions of the system and to treat this portion of the eliluent with sulfuric acid as has been described, the objective again being that the proportion of the scrubber eiiluent which is taken by the line 35 for acidiflcation will be such that its rate of Withdrawal from atraveo-` the systemf shellbe: at, leastsuicient to carry. with inthe; amQllnto sulfate that is formed or absorbedintheother portions of. the system pery unit of time. However, if, a, higher yield of. ammonium sulfate is desired,A vor if the amount of sulfate thatisiormedor; absorbed in other por-` tionsl of the system, larger. than that above exemplified.. a; larger, proportion of; the efiluent can, he removed end acidiedwith sulfuric acid to'form ammonium sulfate andsulfur dioxide byI reaction, with. the; ammonium, sulte ccn'npoundn comprised therein. Alternatively if thefarnount of.A sulfate thatis-.formed or` absorbed inl other portions of thesysteni, is 1 ess. than that above exernpliiied,A the proportion, o the scriibloer. effluent thatis, removedand.- acidined may be sultry.-` stantiallyless..

While the total amount ot sulur dioxide and of ammonium sulfate` recovered accordingv to, the example describedy in connection with Eig- 2y are the same as inthefcase-o theexarnpledeseribed inconnection With'Fg; 1 and v/hilethe require ments` for, supply of' ammonia and sulfuric acid are the same,l the system. exemplied in connestiori with Fig; 2 affords economies as regards the amount of. heat. required as. compared with the system exemplified inconnection with Fig. 2, Thus, in thecase of the-latter system,V the amount of steam required in ter S. of, cunds per pound of. sulfur dioxide recov* is 9.71 pound for the preheater. 1 77 poundsfer. the. stripper 38. and :pound for the' evapieratQr-Crvstallizer 6.05, the total being.- 3556. Pound-s as compared with a total of ir-8 8 pounds the. caso @i the example described in. connection.- witii Eig... 1..

The` Fractie oi this invention is further illus tinted by the indovino enamel@ that is described inconnection. with Eis. This example differs from the examples hereinabove described in con-v nection with Eissl and 2 in that all of the. sulfuricfacid is; added to the eiluent from the scrub-v ber prior, to stripping and diie to this fact still further' economies as regards the requirements for hea-t supply may be afforded. Moreover, acf @ordine to this iurther example advantage is taken with the relatively high concentrations whichv Can be afforded in. the case of absorption of. Suifur dioxide by ammoniaoal solution where? byA en evaporatorcrystallizer is eliminated and the ammQDium sulfate. produced is recovered solely by cooling the residual amincniacal solu-A tion which results from the stripping step. For purposes of comparison the conditions are other. wise the same as hereinabove exemplified in connection with Figs. l and 2. Thus, the temperature and composition of the waste gases entering the scrubber are the same, and the composition of the exit gases from the scrubber is the same. The vaporl pressure of the sulfur dioxide. contained in the waste gases at the above mentioned points with reference to their travel through the scr-uhber is likewise the same and the same quantities of sulfur dioxide are absorbed in the .dif-v ferent zones of the scrubberl and the same proportion of the sulfur dioxide becomes oxidized to sulfate.. The temperatures maintained in the vzones Ha, Hb and Hc likewise aresubstantially the same. As in the previous examples, the amount of ammonia. supplied to the system is .45 mol in the form of a 36% aqueous ammoniacal solution and 0.162 mol of sulfuric acid is sup'- plied. 0.225 m01 of ammonium sulfate isy recovered.

:Insoiar Fier 3 contains the same elements as those @remeber-.e described in connection 18.. withFigs. A1 the saine reference characters have bcenaprlisdrisi icssfliriipri therfjis nwwph in@ encantoticn of available.. i@ 1Q?) 112th@ ammoniacaisoluiion @monasterios l I ffft'h'e Sibf. bjr melma. '3.,' 144,199@ 29X lpoQl-WQI and the coneent ionwodf thetotalainrnonia'ct) Yis of water. encontrado@ O fffsulff? 'is 0-6'161' per vidols oivvaterr o ,ndt-he` value br the 'ratio S/Cisf-Sll 36,6% Qf'ihesplutilon Marins Z011@ H Q of. the bbr hy. i??? Giui lies 'is i'* cyoicdio, the. I LCDY the ll l5 @MW 'a'ml 3.4 of the solution invriezen@ i I C. is 'directed into the rone Hb of the scrubber by the lines 'lY and itL mois ci We??? rom lirici? e'd micio@ zd Hooi @scrubber by ihelinsnf 2 i and I3 together with 15% Q f, the, aqueous. fi-

29.. ai and is, Y

3-8, ci, the. Solution leaving, the, 29.1.18, iin 0f. thescmbbcrbrihe ted" t' the zone li aqueous. ani oriid freni Supply soiree 2 2' 1S als@ d iiaby thel i moniocel S 0f the System is also ed interne ih lineSill/aii ion contains about en the Zones MJ? the ZQIie Hd, the ammonia @o 2115. mle Por IQ ois Qi. wa'ii .i non ofthe .Salies d .Xido )l iS. substantie y 10 mole Per. 101i melf tenth@ conceniiiicih of senate (4) is incl per 1,01) mois'ofviatr and theA value of die tied/.Ci is o 5;

The .effluent rooted by the iiie's 34, a.

directed t0. v

of nie compound eritema lease of suliilr dioxide en@ nium sulfate' The sul from the solution Sciieratiqii iss.

the added sulfur .Sure of. the .Snif-ur #dioxideattire. stripping tern- -nerature which aisnesirablsi about. 225,.o ...the y*stlpptry d8. `the .niiesureiin .the ,stripper v38 zh eing y 1.9 y, atmospheric pressure. Under such conditions .654 mol of sulfur dioxide is taken from the stripper 38 by the line 44 along with 4 mols of water as water vapor and 0.013 mol of ammonia. The water vapor is condensed in the condenser 46 with which the ammonia absorber 45 is'associated and, since it is only desirable to return 3.3 mols of water to the zone Hc of the scrubber by the line 20, 0.7 mol of Water is removed from the system by the line 12. The 3.3 mois of water recycled through the line 20 carries with it 0.013 mol of ammonia combined with 0.013 mol of sulfur dioxide. The balance of the sulfur dioxide, namely, .641 mol, is separated from the condensed Water vapor and is directed by the line 1l to the sulfur dioxide recovery means which is indicated at 49.

The residual ammoniacal solution in the stripper 38 contains the ammonium sulfate which results from oxidation of 9% of the sul'- fur dioxide recovered from the waste gases and ammonium sulfate which results from the addition of the sulfuric acid to the solution entering the stripper. sulfate in the solution is suiiiciently high that, on cooling, crystals of the ammonium sulfate will form. This solution is partially cooled in the heat exchanger 31 in the line 14 which diizo The concentration of ammonium rects the residual ammoniacal solution from the stripper 38 to the crystallizer 15 wherein the residual ammoniacal solution is further cooled as by the supply of cooling water through line 16. The slurry of ammonium sulfate which becomes crystallized in the crystallizer 1'5 together with the mother liquor is passed to the centrifuge 11 by the line 18 from which the crystallized ammonium sulfate is taken by any suitable means indicated by the line 19 to a suitable means for collectingY the recovered ammonium sulfate which is indicated at 80. The amount of ammonium sulfate thus recovered, namely .225 mol is that which is produced in the system as the result of oxidation of sulfur dioxide in the scrubber and as the result of the addition of sulfuric acid. The mother liquor from the centrifuge 11 is directed as aforesaid into the zone Ila of the scrubber by the lines 13, 3l and 28. In this recycled ammoniacal solution the concentration of available ammonia (Ca) is 10.65 mois per 100 mols of water, the concentration of sulfur dioxide (S) is substantially 9.73 vmols per 100 mols of water', the concentration of sulfate (A) is 5.7 mols per 100 mois of water and the value of the ratio S/Ca is 0.917.

In connection with the example of the practice of this invention which has been described in connection with Fig. 3 the entire amount of the sulfur dioxide which is produced, namely, 91% ofthe sulfur dioxide removed from the waste'gases, is separated from the solution in the stripper'38, and the other 9% is oxidized and is withdrawn as ammonium sulfate. However, as in the 'case of the examples described in con'- nection with Figs. 1 and 2 a portion of the sulfur dioxide so recovered is to be regarded as having been produced by the reaction between ammonium sulte compound and sulfuric acid "with formation of ammonium sulfate and sulfur di# oxide and another portion of the sulfur dioxide is to be regarded as recovered by stripping it from the ammoniacal solution along with water vapor.

In order to aiord a comparison between the examples of this invention which have been described in connection with Figs. 1, 2 and 3 like 20 conditions have been described with reference to the composition and temperature of the waste gases, the amount of ammonia and sulfuric acid supplied to the system and the amount of re` covered sulfur dioxide and ammonium sulfate. Ihus, in connection with each example 25 mois of sulfuric acid are required per 100 mols of sulfur dioxide produced and 0.66 ton of ammonium sulfate are produced per ton of sulfur dioxide recovered from the waste gases. Each of the examples of the practice of this invention which have been hereinabove described afford the advantages that have been mentioned hereinabove in connection with the employment of the system described in connection with Fig. 1 as compared with the release of recovered sulfur dioxide solely by acidication with sulfuric acid. As between the methods described inA connection with Figs. 1, 2 and 3 that utilizing the system described in connection with Fig. 3 involves'the lowest heat requirements. Thus, according to the example described in connection with Fig. 3 no steam is required for an evaporator-crystallizer as is the case respecting the examples described in connection with Figs. l and 2. According to the example described in connection with Fig. 3 0.56 pound of steam is required in the preheater per pound of sulfur ydioxide recovered and only 1.85 pounds of steam per poundof sulfur dioxide recovered is required in the stripper, the total steam requirements thus being only 2.41 pounds of steam per pound of sulfur dioxide recovered. This amount of steam required may be compared with the total of y4.88 pounds of steam according to the example hereinabove described in connection with Fig. 1 and the total of 3.36 pounds of steam per pound of sulfur dioxide 'recovered in the case of the example hereinabove described in connection with Fig. 2. While the example of the practice of this invention which has been described in connection with Fig. 3 has the advantage of having the lowest steam requirement, nevertheless, the practice of this invention which has been described in connection with Fig. 3 is somewhat less ilexible in that in order to obtain crystallization of ammonium sulfate solely by cooling the residual solution formed in the stripper, it is necessary to maintain relatively high rsolution concentrations in the eiiiuent which is taken from the scrubber and which is subjected to stripping after acidication with sulfuric acid.

The practice of this invention as illustrated in connection with any of Figs. 1 to 3 has the advantage of exibility as regards the relative proportions of sulfur dioxide and ammonium sulfate which may be produced in the recovery of sulfur dioxide from waste gases. Each of the examples of the practice of this invention which has been described hereinabove has illustrated the recovery of the sulfur dioxide with an accompanying low rate of production of ammonium sul'- fate as a by-product. In this way the amounts of ammonia and of sulfuric acid required to be supplied to the system likewise can be minimized. Moreover, the amount of recovered sulfur dioxide which must be converted to sulfuric acid for the purpose of maintaining the supply of sulfuric acid to the system can be held to a low value with resultant lessening of the costs incident to the provision and operation of a sulfuric acid plant as part of the overall system and with resultant increase inthe proportion of the recovered'sulfur dioxide that does ynot have to be acidl` andi that can` be" taken as such from the system; However; depending onsuch mattersas the market value ofl ammonium sulfate and ofA sulfurdioxidearecovered as-suchi as well as the availability ofV markets for such products, each'. of the systems above described in connection with4k Fig-s; I; 2' andiv 32 is; such thatl the relative proportions of ammonium sul-fate andofsulfur dioxide l' can `be varied' byf increasing or decreasing therelative proportions of the portions of ammoniumfsulfite compound contained in the eiilu'` ent from the; scrubber whichy are subjected tof acidification with' sulfuricl acidv to` fornr am-v monium sulfate and: sulfur dioxide andhavesulfur dioxide separated therefrom as the result-cf'. stripping;

While` the practice oi' this; invention; has L been described in.` connection vwith certain speciflc examplesl it is apparent: thatthe` practice of this invention: may be varied in respects other than those-mentionede.hereinabcve;. Thus, inthe recoveriy of. sulfur'dioxide'from waste gasesgthefcoii-l centration'siy of. the.. ammoniacal. solution brought into contact with the. Waste gases may be: varied andi such. variation mayI be; advantageous in the casemf Waste. gases: which` may-.be at. a different temperature; or which may contain different: amounts of' sulfur dioxide. or` of. water: vapor. Moreover; whileatheabsorptiorr of; the sulfur dioxide; from; wastel gases. has been described in connectiomwith such absorption. in. threezones wherein .thelconnentratiort of the available am.-A moniaa. inLtheiarmnoniacal-.solution is diierent eachazone; the: absorption maybe caused to. take plac'e'..in1a greater or lesser; number of zonesv or even` inv ai single zone.` However, it is. preferable to.l carryfout. the absorptionf in; at least two zones wherein. the, concentration ci the available ammania; in` the ammoniacal, solution: isdecreased inthe direct-ionof; thepassageof thewaste gases from one, Zone; to: the next soas: to obtain high concentrations; of. absorbed` sulfur dioxide in' the euentlfrom. the schubben, efficient recovery of the .sulfur dioxide and 1 low, ammonia losses..

Ordinarilyit is preferable to maintain thetempesatural at which absorption of4 sulfur dioxide is carriedout-in the-scrubberat-as low a tempera-- ture aspossible'inasmuchas. under such conditionst-the'capacity of the ammoniacal solutionisgreatenand the scrubbing,r efficiency isgreater particularly asregards accomplishing a good re. oovery of sulfur dioxide.l from dilute gasesl with@V out` increasingthee losses of ammonia to the Stacie gasestd` an objectionable extent. However, waste` gases oftenl contain a substantial amount of water' vapor resulting from the combustion of hydrogen andv from evaporation. of Water contained in. a fuel such as` coal, and this issigniiicant. for it is the wet bulb temperature ofthe waste gasesY which automatically establishes the temperature of the` absorption inthe scrubber.. For. example,V the:wet bulb temperature is ap- Eroximately 127 F.. for stack gases at 360 F. containing 0.05 pound of water vapor per pound cidry gas. Actually, however,; the equilibrium saturation temperature. of the scrubbing solution may-beslightly highene. g., about F..higher, because of the lower vapor pressure of water in equilibrium with. a scrubbing solution` for rela-,- tively high solutionconcentrations. Accordingly, in`A ord/ente. obtain increasedscrubbing efficiency andA increased capacity of'. the scrubbingsolution, deh-umidication. and cooling ofthewaste gases,v as'b'y direct contact with a large volume of waterin: a.l pre'cooleir, r mambo-justified im somecasese andv the: foregoing' examples; off; the practice ct this invention havevbeen. connection; waste gasesfthusrdehumidiedi and cooled... Have@- ever, the gasesneed notlrrecessamily hedehum-idt-- edand percooledin:practicing'this;invention.. i

The particulars conditions mar-mtained ine'thef; system maylbe-variedfroml those described heres inabove iny connection withftheforegoing descri'pa tioxr ofv typical examplesof the practiceA offthisr invention.. Important: factorsy this: regard; which aiiectr the conditions desirabl'y maintained; are the concentration oftlfle sulfur -dioxideinithe waste gases and the temperatureiatwhich'their absorption iseffected by contact with the ainmoniacal solution. The number orthescrubbing zones is likewise important as regards the conditions maintained in 'eachzonea Ordinarily it is desirable'tnfmaintain the capacity of the -scrubbing solution relatively high particularly if the concentration of the sulaurv dioxide in, the waste gases. is low.:- and: iffthe temperature of theabsorption is relatively high, subject, however, to the desirability of avoiding concentrations which would result ini. undesirably-high ammonia losses to the waste gases.` The capacity of the ammoniacal solution ,also is significant as regards the amount thereofv thatissrecireulatedinto, contact.

with the waste gases Alturtherfdisadvantage in connection. with.. employment. of.vv relatively low concentrations, in. the ammoniacal' solution.` used is that in. effecting"thereceveryoty ammonium sulfate by crystallization the evaporation of more water is required'. The capacity. and degree of absorption of sulfur dioxide in an ammoniacal solution also .desirably aiects the amount of sulfuric acid requiredfor reaction toform ammonium sulfate andsulfur dioxide, for it is apparent that requirementsftor. sulfuric; acid inthis regard decrease as theratio SC increases Theamount of. sulfuricfacid-l employed likewise is dependent upon thev proportionfoi ammoniumsulfate which is produced by the process; as ,the result of addi-ng sulfuric acid for reaction. withfammonium sul,- iite compoundl` tc.- produce. ammonium sulfateandI sulfur dioxide- Generally it. is; desirable that. the proportion, of. sulfur. dioxide whichl isyproa ducedby the reaction, ot ammonium. sulztl-tefcom` pound. comprised iii-effluent from thescrubber with. sulfuric. acidtoV form..- ammonium sulfate and sulfur dioxide' be at least. about 35.172,.y by weight of the total sulfur dioxide obtained, by said reaction plus the additional sulfur dioxide separated from.' ammoniumsul't'e compound'`v by a, stripping. Howeveigsdepending upon the amount.

of ammonium sulfate which it` may be. regarded` as desirable to produce, theY amount of sulfuricy` acid' used, and, by like token the amount of' am# monium sulfate recovered',` may be increased and'. in such. case. the proportion, oflthe sulfur, dioxide.r producedv by` reaction of. ammoniumsultle compound with sulfurioacid would bei. increased'. in relation to the total. sul'f'ur dioxide obtained.` However,.it is usually preferable that. the proportion of' sulfur dioxide produced' by reactionof` ammonium sul'te. compoundlwith sulfuric acid bey not greater,A than about '15 by. weightY off the" total sulfur dioxide. obtained by reaction with thev sulfuric acid'- plus the additional sulfur dioxide obtained by stripping. These., comments are ap.- plicable tovany ofthe modifications of this. in.- vention whichhave been` describedlhereinabove..

More generallmparticularly asy regards the ab? sorption othe sulfur. dioxide from the wastev gasesand. the .character ofi the. solution obtained These equations hold when there is no sulfate or salt other than ammonium sulte-bisulte. If sulfate is present in the solution from the absorption of sulfuric acid vapor or oxidation of part of the sulte, the equations become ors-T211 (6) Grou-Saai PNH-Absozfi Y (7),.

I'he constants M and N are functions of the absolute temperature UK.) as shown.v The equations give the partial pressures in millimeters of mercury and in this equation P* is the equiiibrium partial pressure with the solution, Ct is the total concentration of ammonium ions in the solution in terms of mols per 100 mols of water, f

S is the concentration of sulfur dioxide in solution in terms of mols per 100 mols of water, A is the concentration of sulfate ions (or other eduiw` alent bivalent ions) in solution in terms of mols per 100 mols of water, and T is absolute temperature, degrees Kelvin.

I claim:

1. A process for the recovery of sulfur dioxide from Waste gases wherein aqueous ammoniacal solution is directed into contact with said gases, sulfur dioxide Ycontained in said gases is absorbed by said solution by reaction with ammonia contained therein to form ammonium sulte compound dissolved in said solution and eiiiuent solution is separated from said gases which comprises the steps of reacting a portion of the am-` monium sulfite compound comprised in said'ef fluent solution with sulfuric acid to form am'- monium sulfate and sulfur dioxide by adding sulfuric acid to eiliuent solution containing said portion of said ammonium sulfite compound, sep-V arating from the solution sulfur dioxide so formed leaving residual solution containing ammonium sulfate, collecting the separated sulfur dioxide, crystallizing ammonium sulfate from said residual solution, collecting and recovering said crystalized ammonium sulfate, stripping sulfur dioxide irom a second portion of said ammonium sulte compound comprised in said eiuent solu-Y tion by heating eiluent solution containing said second portion of said compound to separate sulfur dioxide therefrom leaving residual ammoniacal solution, collecting the so separated sulfur dioxide, and recycling said residual ammoniacal solution so as to be comprised in said ammoniacal solution directed into contact with said waste gases.

2. A process according to claim 1 wherein the proportion of sulfur dioxide produced -by reaction of ammonium sulte compound comprised in the eiiiuent solution with sulfuric acid is a minor proportion by weight of the total sulfur dioxide obtained from said eiiiuent solution by said reaction and by stripping sulfur dioxide from ammonium sulte compound comprised in said solution. l

3. A process according to claim 1 whereinvthe proportion ofsulfur dioxide produced by reaction of ammonium sulte compound comprised in the eiiiuent'solution with sulfuric acidconstitutes from about 35% to about 75% by Weight of the total sulfur dioxide obtained from said eiuent solution by said reaction and by stripping sulfur dioxide from ammonium sulte compound comprised in said solution.

. 4. A process for the recovery of sulfur dioxide from Wastev gases which comprises directing aqueous ammoniacal solution into contact with the waste gases in at least two zones through which the gases are sequentially passed from a' rst zone to a second zone and in which sulfur dioxide contained in said gases is absorbed by said ammoniacal solution by'reaction with ammonia contained therein to form ammonium sulte compound dissolved in said solution, maintaining the concentration of ammonia contained in said ammoniacal solution and available for said reaction with the sulfur dioxide in said gases substantially diierent in said zones with substantial decrease in said concentration in the direction of sequential travel of said gases through said zonesv so that said concentration in Vsaid first zone is substantially greater than said concentration in said second ozone, separating' from said gases effluent solution containing the' am-A monium sulfite compound formed by the absorptionA of sulfur dioxide therein from said gases, reacting a portion of the ammonium sulfite cornpound in said effluent solution with sulfuric acid to form ammonium sulfate and sulfur dioxide by adding sulfuric acid to eiiluent solution containing said portion of said ammonium sulte compound, separating from the solution sulfur dioxide so formed leaving residual solution containing ammonium sulfate, crystallizing ammonium sulfate from said residual solution,"collect ing and recovering said crystallized ammonium sulfate, stripping sulfur dioxide from a second portion of said ammonium sulte compound comprised in said effluent solution by heating efuent solution containing said second portion of said compound to separate therefrom sulfur dioxide commingled with water vapor leavingV residual ammoniacal solution, separating the sulfur dioxide from said YWater vapor by condensa-` tion of said Water vapor, collecting the sepafrated sulfur dioxide, recycling condensate resulting from the condensation of said Water Vapor so as to be comprised predominantly in ammoniacal solution directed into contact with said gases in said second zone, and recycling said residual ammoniacal solution so as to be comprised predominantly in ammoniacal solution di-` rected into contact with said gases in said 'rst zone.

5,-. A- process for the recovery of sulfur dioxide from waste` gases whereinaqueous ammoniacal solution is; directed into contactl with said gases, sulfur dioxidecontained in said. gases' is absorbed by said solution by reaction with ammonia` contained therein. to. form ammonium sulfite compound dissolved in said solution and ellluent solutionis separated from said gases which comprises the steps of heatingza rst portion of said efiiuent solution to separate sulfur dioxide and Water vapor therefrom leaving residuall ammoniacal solution, recycling said residual ammoniacal solution so-as to be comprised in said' ammoniacal solution directed into contact with said waste gases, separating sulfur dioxidefrom; said water vapor by condensing said water vapor', collecting said: sulfur dioxide, mixing sulfuric acid with a second portion of said eiiluent solutiony for reaction with ammonium sulte compound contained therein to form ammonium sulfate. and sulfur dioxide, separating sulfur dioxide so produced from said second portion of the solution leaving residual solution containing ammonium sulfate, and .collecting the separated sulfur dioxide.

6. A process according to claim wherein said second portion of said eiiiuent solution to which said sulfuric acid is added is withdrawn from the system at a rate which is at least suiicient tocarry with it any amount of sulfate that is formed or absorbed in other portions of the systemi 7. A process according. to claim 5 wherein the proportion of sulfur dioxide produced by reaction of armnoniuml sulte compound comprised in said second. portion of said eiiiuent solution with sulfuric acid constitutesl from about 35% tolabout 75% by weight of the total sulfur dioxide obtained from said second portion of said effluent solution. by said.- reaction. and obtained by stripping sulfur dioxidefrom ammonium sulfite compound comprised in said firsty portion of said etliuent solution.

8A A processfcr. the recovery: of sulfur dioxide from wastegases which comprises directing aqueousammoniacal; solution into. contact with the waste gases. in at leasttwo. zonesI throughl which the gases are sequentially passed from a iirst zone to a. second zone and inwhich sulfur dioxide contained. in said gases is absorbedl by said ammoniacal solution by reaction with ammonia contained therein to form. ammonium sulte; compound dissolved in said solution., maintaining the concentration of ammonia contained in said ammoniacal solution and available for said reaction with the sulfur dioxide, in. said gases. substantially greater in said zones with substantial decrease in said concentration in the direotionvof sequen tial. travel of. said gases through :said zonesl so that said concentrationin said. firstzone is substantially greater than said concentration in said second zone, :separating from said gases effluent solution containing the ammonium suliite compound .formed by absorption of sulfur dioxide from said, gases,l heating a rst. portion of said effluent solution to separate sufur dioxide and water vapor therefrom leaving residual ammoniacal solution, recycling said residual ammoniacal solution so as to be comprised predominantly in ammoniacal solution directed into contact with said gases in said firstv zone, separating the sulfur dioxide so produced from said water vapor by condensing said water Vapor, collecting the separated sul'fur dioxide, recycling condensate resulting from the condensation of said water vapor so as to be comprised predominantly in ammo- 26e niacal solution directed into contact with said gases in saidv second zone, mixing sulfuric` acid with a secondv portion. of. said eiiluent solution which is substantially less. by volume than, said rst portion so as to react with the ammonium sulfite compound contained in` sa-id second portion to form ammonium sulfate and sulfur dioxide, separating suliur dioxide so formedr from the solution leaving residual solution containing ammonium sulfate; crystallizing ammonium sulfate from said residual ammonium, sulfate-containing solution and collecting and recovering said crystallized ammonium sulfate. l

9; A process for the recovery ofsulfur dioxide from waste gases wherein aqueous ammoniacal solution is directed into contact with said gases, sulfur dioxide contained in said gases isabsorbed by said solution by reaction with ammonia contained therein to form ammonium sulfite compound dissolved in said solution and eiiluent solution isseparated from said gases which comprises the steps of adding sulfuric acid to a iirst portion of said effluent solution to raise the vapor pressure of sulfur dioxide contained therein by reacting part of the ammonium suliite compound contained in said iirst portion of the eiiiuent solution so as to form ammonium sulfate and sulfur dioxide, heating the` resultant solution to strip therefrom sulfur dioxidel formed by said reaction of ammonium sulte compound with the said sulfuric acid together with additional sulfur dioxide and water vapor leaving residual ammoniaoal solution, recycling said residual ammoniacal solution so as to be comprised in ammoniacal solution directed into contact with said waste gases, separating the sulfur dioxide from the water vapor by condensing the Water vapor, collecting the so-separated sulfur dioxide, adding sulfuric acid to asecond portion ofy said eiliuentv solution so as to react with ammonium sulte compound contained therein to-form ammonium sulfate and sulfur dioxide, separating' the sulfur dioxide so produced leaving residual ammonium sulfatecontaining solution and collecting thel separated sulfur dioxide.

10. A process according to claim 9 wherein said second portionv of said eilluent solution to which said sulfuric acid is added is withdrawn from the system at a rate which is at least sufficient to` carry with it any amount of sulfate that is formed or absorbed in other portions oi' the system per unit of time.

11. A process according to claim 9 wherein the proportion of sulfur dioxide produced by reaction of ammonium sulte compoundvv contained in said first and second portions of the eiiiuent solution withsulfuric acid constitutes about 35% to about '75% by weight. of the total sulfur dioxide obtained by said reactionV plus the additional sulfuridioxideseparated from said rst portion of the eiiiuent solution by stripping suliur dioxide from said iirst portion of the eiiluent solution.

l2. A process for the. recovery of sulfur dioxide from wastev gases which comprises directing aqueous ammoniacal solution into contact with the waste gases. in at least two zones through which the gases are sequentially passed from a first zone to a second zone and' in which sulfur dioxide contained in said gases. is absorbed by said ammoniacalsolution by reaction with ammonia contained therein to form ammonium Sulfite compound dissolved in said solution, maintaining the concentration vof ammonia con,- tained in said ammoniacal solution and available for said reaction with the sulfur dioxide in said gases substantially diiTerent in said zones with substantial decrease in saidconcentration in the direction of sequential travel of said gases through said zones so that the said concentration in said first zone is substantially greater than said concentration in said second zone, separating from said gases eilluent solution containing the ammonium sulte compound formed by the absorption of the sulfur dioxide therein from said gases, adding sulfuric acid to a rst portion of said eluent solution so as to increase the vapor pressure of the sulfur dioxide therein by reaction with a partof the ammonium sulte compound contained in said rst portion of said eiiiuentV solution, heating the so-acidied first portion of the solution so as to strip therefrom sulfur dioxide resulting from reaction with the added sulfuric acid together with additional sulfur dioxide and water vapor leaving residual ammoniacal solution, separating the stripped sulfur dioxide from the water vapor by condensing the Water vapor, collecting the so separated vsulfur dioxide, recycling condensate resulting from the condensation of said Water vapor so as to be comprised predominantly in the ammoniacal solution directing into contact with said gases in said second zone, recycling said residual ammoniacal solution so as to be comprised predominantly in ammoniacal solution directed into contact with said gases in said rst zone, adding sulfuric acid to a second portion of said eiuent solution for reaction with the ammonium sulnte compound contained in said second portion of the eiuent solution to form ammonium sulfate and sulfur dioxide, separating the so-produoed sulfur dioxide from the solution leaving residual solution containing ammonium sulfate, crystallizing ammonium sulfate from `said ammonium sulfate-containing residual solution, and collecting and recovering the ammonium sulfate so produced, said second portion of said efluent solution being substantially less than said first portion of said effluent solution and, containing an amount of ammonium sulfate which is at least Vequal to the amount of ammonium sulfate formed by adding sulfuric acid to said rst portion of said eiiiuent solution.

13. A process for the recovery of sulfur dioxide from Waste gases wherein aqueous ammoniacal solution is directed into contact with said gases, sulfur dioxide contained in said gases is absorbed by said solution by reaction With ammonia contained therein to form ammonium sulfite compound dissolved in said solution and eiiiuent solution is separated from said gases which comprises the steps of adding sulfuric acid to said effluent solution for reaction lwith part of the ammonium sulfite compound,l contained therein to form ammonium sulfate'and sulfur dioxide, heating the acidifled solution so as to strip therefrom sulfur dioxide formed by reaction between the ammonium sulfite compound and the added sulfuric acid together'with additional sulfur dioxide and Water vapor leaving residual ammoniacal solution containing ammonium sulfate, separating the stripped sulfur dioxide from the water vapor by condensing the VWater vapor, collecting the sulfur dioxide so produced, cooling the residual ammoniacal solution containing ammonium sulfate to crystallize ammonium sulfate therefrom leaving residual aznmoniacal solution, separating vthe crystallized ammonium sulfate from said residual amrnQlLi- 28' acal solution, and recycling the residual ammoniacal solution separated from the crystallized ammonium sulfate so as to be comprisedy in the ammoniacal solution directed into contact with said waste gases.

14. A process according 'to claim 13 wherein the proportion of sulfur dioxide produced byv reaction of ammonium sulfate compound comprised in the effluent solution with sulfuric acid is about 35% to about 75% by weight of the total sulfur dioxide obtained by said reaction plus the additional sulfur dioxide separated from the eiiiuent solution by stripping sulfur dioxide therefrom.

15. A process for the recovery of sulfur dioxide from Waste gases which comprises directing aqueous ammoniacal solution into contact with the Waste gases in at least two zones through which the gases are sequentially passed from a first zone to a second zone and 'in Which sulfur dioxide contained in said gases is absorbed by said ammoniacal solution by reaction with ammonia contained therein to form ammonium sulfite compound dissolved in said solution, maintaining the concentration of ammonia contained in said ammoniacal solution and available for said reaction With the sulfur dioxide of said gases substantially different in said gases with a substantial decrease in said concentration in the direction of sequential travel of said gases through said zones so that said concentration in said first zone is substantially greater than said concentration in said second zone, separating from said gases eiliuent solution containing the ammonium sulte compound formed by the absorption of sulfur dioxide from said gases, adding to said efuent solution sulfuric acid for reaction with part of the ammonium sulte compound contained in said eiiiuent solution to form ammonium sulfate and sulfur dioxide, stripping from the acidiied solution the sulfur dioxide produced by reaction Withvthesulfuric acid together with additional sulfur dioxide and Water vapor leaving residual ammoniacal solution containing ammonium sulfate, separating the stripped sulfur dioxide from the Water vapor by condensing the Water vapor, collecting the sulfur dioxide so produced, recycling condensate resulting from the condensation of said Water vapor so as to be comprised predominantly in ammoniacal solution directed into contact with said gases into said second zone, cooling the residual ammoniacal solution containing ammonium sulfate so as to cause crystallization of am.- monium sulfate therefrom, separating crystallized ammonium sulfate from the ammoniacal solution, and recycling the ammoniacal solu-v tion from which the ammonium sulfate 'crystals have been separated so as to be comprised predominantly in ammoniacal solution directed into contact with said gases in said first zone,

References Cited in the rlle of this patent UNITED STATES PATENTS 101,738 Australia .-Aug. 12, 1937 

1. A PROCESS FOR THE RECOVERY OF SULFUR DIOXIDE FROM WASTE GASES WHEREIN AQUEOUS AMMONIACAL SOLUTION IS DIRECTED INTO CONTACT WITH SAID GASES, SULFUR DIOXIDE CONTAINED IN SAID GASES IS ABSORBED BY SAID SOLUTION BY REACTION WITH AMMONIA CONTAINED THEREIN TO FORM AMMONIUM SULFITE COMPOUND DISSOLVED IN SAID SOLUTION AND EFFLUENT SOLUTION IS SEPARATED FROM SAID GASES WHICH COMPRISES THE STEPS OF REACTING A PORTION OF THE AMMONIUM SULFITE COMPOUND COMPRISED IN SAID EFFLUENT SOLUTION WITH SULFURIC ACID TO FORM AMMONIUM SULFATE AND SULFUR DIOXIDE BY ADDING SULFURIC ACID TO EFFLUENT SOLUTION CONTAINING SAID PORTION OF SAID AMMONIUM SULFITE COMPOUND, SEPARATING FROM THE SOLUTION SULFUR DIOXIDE SO FORMED LEAVING RESIDUAL SOLUTION CONTAINING AMMONIUM SULFATE, COLLECTING THE SEPARATED SULFUR DIOXIDE, CRYSTALLIZING AMMONIUM SULFATE FROM SAID RESIDUAL SOLUTION, COLLECTING AND RECOVERING SAID CRYSTALIZED AMMONIUM SULFATE, STRIPPING SULFUR DIOXIDE FROM A SECOND PORTION OF SAID AMMONIUM SULFITE COMPOUND COMPRISED IN SAID EFFLUENT SOLUTION BY HEATING EFFLUENT SOLUTION CONTAINING SAID SECOND PORTION OF SAID COMPOUND TO SEPARATE SULFUR DIOXIDE THEREFROM LEAVING RESIDUIAL AMMONIACAL SOLUTION, COLLECTING THE SO SEPARATED SULFUR DIOXIDE, AND RECYCLING SAID RESIDUAL AMMONIACAL SOLUTION SO AS TO BE COMPRISED IN SAID AMMONIACAL SOLUTION DIRECTED INTO CONTACT WITH SAID WASTE GASES. 